1. Field of the Invention
The present invention relates to an emergency robot shutdown circuit, and more particularly to an emergency robot shutdown circuit for reliably stopping a movable robot component which has traveled beyond a certain range or suffered an overtravel condition.
2. Description of the Related Art
Robots that are program-controlled by numerical control apparatus are equipped with various emergency shutdown circuits for better safety in working environments. One such emergency shutdown circuit is designed to operate when it detects an overtravel condition of a robot. In the case where the emergency shutdown circuit is combined with a shaft rotatable on a base, such an overtravel condition is established by the emergency shutdown circuit as a certain inoperable angular range in which the shaft is not rotatable. The overtravel range is not necessarily associated with a rotatable shaft. Generally, an emergency robot shutdown circuit is arranged such that if a movable shaft is about to move beyond a normal operation range, then a limit switch presses a dog, forcibly shutting off the supply of power to a robot moving mechanism thereby to stop the robot.
In recent years, moving speeds or rotational speeds which are given as command speeds to robots have been increasing for higher efficiency. It is more and more difficult to reliably stop a movable robot component in an overtravel condition merely with an overtravel signal which is generated by a limit switch when the movable robot component has traveled beyond a normal operation range.
FIG. 3 of the accompanying drawings shows in perspective a robot having a rotatable shaft 3 on a fixed base. A limit switch 1 which defines an inoperable angular range is fixedly mounted on the base. A dog 2 of a certain length is mounted on the rotatable shaft 3 for engaging the limit switch 1 to cause the limit switch 1 to output an overtravel signal.
FIG. 4 of the accompanying drawings shows a conventional emergency robot shutdown circuit.
As shown in FIG. 4, a dog for detecting an overtravel condition is located in a limit position of an operable range of a movable component of a robot 10. The dog and the limit switch 1 jointly serve as a detecting circuit 11 for producing an overtravel signal. When the limit switch 1 presses the dog and is turned off thereby, a relay R4 is de-energized, opening a make contact (normally open) r4 thereof thereby to de-energize a relay R5 which is connected in series with the make contact r4.
A teach control console 12 has an emergency shutdown button connected in series with the make contact r4. Therefore, the relay R5 can also be de-energized when the emergency shutdown button is pressed. An overtravel cancel switch 13 is connected parallel to the make contact r4. The relay R5 can be energized when the overtravel cancel switch 13 is closed.
When an overtravel signal is detected, the relays R4, R5 are simultaneously de-energized, opening make contacts r5a, r5b, r5c of the relay R5 in a power cutoff circuit 14. Now, a power supply 15 and an amplifier 16 for actuating the robot 10 are disconnected from each other, thus shutting off the robot 10 in case of emergency. When the overtravel cancel switch 13 is closed, the power is supplied to the movable component of the robot 10 to return the same from the overtravel condition even if the limit switch 1 is pressing the dog.
In the event that the movable component of the robot 10 moves beyond the dog, however, the relays R4, R5 are energized again, and the power is supplied to the movable component of the robot 10 which is in the inoperable range before the overtravel cancel switch 13 is closed. Stated otherwise, even after the movable component of the robot 10 has entered the inoperable range, the movable component keeps moving due to a delay of the response to the overtravel signal or an inertia of the movable component. As a result, the movable component of the robot 10 is not reliably stopped only with the overtravel signal produced by the detecting circuit 11.
The above problem can be solved by elongating the dog depending on the speed of movement of the movable component of the robot 10, thereby maintaining a certain period of time in which the overtravel signal is sustained. Since the length of the dog limits the normal operation range of the movable component of the robot 10, however, the length of the dog is physically limited by the size of the robot 10. If the command speed for moving the robot 10 is increased, then the movable component of the robot 10 cannot reliably be stopped when in an overtravel condition.